These experiments will chart the connections between the inferior colliculus (IC), medial geniculate body (MGB) and auditory cortex (AC), three structures essential for normal hearing. This will enable us to propose hypotheses about the nature of the tectothalamocortical transformation. Three studies are proposed. First, we will analyze connections within the IC that permit its several subdivisions to communicate, with emphasis on structures other than the central nucleus. We will then document the types of IC neurons containing gamma-aminobutyric acid in immunocytochemical analyses. Second, we will trace IC connections to the MGB using anterograde and retrograde tracers. This aim contrasts and compares the projection patterns of IC subdivisions with their MGB targets, revealing more fully the thalamic projections of midbrain souces. Finally, we will label and identify the AC cells that descend to the IC and MGB since these pathways may be important in processing biologically salient sounds. These experiments use anesthetized rats and cats to document how these structures interact. The cat is also used in collaborative physiological-anatomical studies. The ultimate goal of this work is to create plausible, principled, and data-based biological models of auditory midbrain, thalamic, and cortical connectivity that incorporate specific projections and local circuits in each of their subdivisions. There is reason to believe that the several subdivisions of the IC each have a unique role to play in hearing, in multimodal integration, and in auditory relations to the limbic system. Placing these roles in a functional hierarchy can clarify the relations between hearing and behaviors such as startle reflexes and audiogenic seizures, processes which likely depend on interactions between auditory midbrain, thalamus, and cortex for their expression. Such models precede more refined theories of auditory system relations with the limbic and motor systems, to which hearing is related closely by connections and function. Damage from stroke, aging or traumatic brain injury can cause deficits in hearing and in speech comprehension. Animal models are an essential step towards understanding effective clinical interventions. At present only hearing aids and cochlear prostheses can be used to ameliorate deafness. It may ultimately be possible to craft central prostheses that could be implanted in IC, MGB, and AC to stimulate the brain directly in attacking the problem of hearing loss. [unreadable] [unreadable] [unreadable]